The invention relates generally to contact pads in integrated circuits, and more particularly to oxide-free bond pads.
Bond pads are electrical terminals which connect an integrated circuit die or chip to the electrical system outside of the chip. The electrical connection is normally made by bonding electrical leads to the bond pad. The chip can then be connected to a larger circuit, such as a printed circuit board (PCB), with the leads making contact with the outside system.
The bond pads are integrally connected to metal lines or runners within the die, which are typically formed of a metal such as aluminum, aluminum-silicon eutectic, aluminum-copper alloys, or polysilicon. The bond pads themselves are also typically formed of aluminum or an aluminum alloy, which is highly conductive and relatively inexpensive. Unfortunately, aluminum or aluminum alloy readily oxidizes to form aluminum oxide. The aluminum oxide is not conductive, and it therefore increases the overall resistivity of the system. Increased resistivity, in turn, leads to slower signal propagation.
Conventionally, aluminum oxide is removed with a reducing agent in several separate steps. The chip is exposed to atmosphere between steps, and the exposed metal spontaneously oxidizes, impairing the conductive connection. Even the short exposure between oxide cleaning and sealing the bond pad results in aluminum oxide formation between the metal and sealant.
There is thus a need for a method of avoiding oxide on the surface of a contact pad.
In view of this need, the present invention provides a method and apparatus for providing conductive passivation on contact pads, such as bond pads.
In accordance with one aspect of the invention, a method is provided for plating a conductive layer in an integrated circuit. The method includes immersing the integrated circuit in a cleaning fluid. The integrated circuit is then transferred from the cleaning fluid to a plating fluid, without exposing the integrated circuit to air.
In an illustrative embodiment, such transfer is performed directly from one liquid phase to another. The cleaning fluid represents a first liquid phase, preferably an oxide etch bath, and the second liquid phase forms a protective layer over the insulating material which surrounds the contact pad. The plating fluid is in yet a third liquid phase, containing a conducting monomer in solution. This forms a monomer layer over the conductive layer, which is later polymerized to form a conductive polymer. The integrated circuit sequentially moves between the first and second phases, and between the second and third phases, without passing through air. As will be understood by the skilled artisan, such an arrangement enables sealing the underlying conductive layer of the contact pad, which may be susceptible to oxidation, immediately after oxide removal. Nether oxide nor other contaminants have the opportunity to form on the conductive layer between steps, which would hinder electrical contact between the contact pad and outside circuits.
In accordance with another aspect of the invention, an apparatus is provided for sequential processing with two or more liquid solutions. The apparatus includes a water-tight tank with an upper portion and a lower portion. The upper portion is divided into at least a first side and a second side by a water-tight barrier. The lower portion is open to and extends beneath both the first side and the second side.
This apparatus is particularly useful for the illustrated process, where one side of the upper portion holds an oxide cleaning agent (e.g., 1% NaOH, density about 1.0 g/cm3) and the other side of the upper portion holds a conducting monomer in solution (e.g., pyrrole, density less than about 0.99 g/cm3). The barrier separates the cleaning solution from the monomer solution. The lower portion holds a relatively more dense solution for forming a protective layer (e.g., siliconizing solution, density about 1.09 g/cm3), ensuring that the phases are naturally separated by gravity.